Method of producing solid carbon dioxide



Sept. 25, 1934. J. s. BELT ET AL 1,974Q791 y METHOD oF PBODUGI'NG SOLIDCARBON DoxIDE AIITTORNEY.

sept-.25,1934 J,S,BELTET AL. 1,974,791

, METHOD OF PRODUCING SOLID CARBON DIOXIDE Filed Jan. so, 195o 2sheets-sheet 2 efose/f/ 3- 5955 INVENTORS.

Ur BVM/W ATTORNEY.

Patented Sept, 25, 1934 Mn'rnon or rnonucmo soLm oaaoN fmoxmE Joseph S.Belt, `Amarillo, Tex., and Hamilton P.

Cady, Lawrence, Kans., assignors, by direct and 'f mesne assignments, toJ. S. Belt Natural Besource Corporation, Phoenix, Ariz.

implication January so, 1930, serai No. 424,655

H 2 claims. (cree- 121) greatly increased', and whereby the coolingofthe Icompressed and expanded gases may. be eco- The present inventionrelates to quantity production of solidified carbon dioxide orcompressed carbon dioxide snow, such as is now commercially used to someextent for refrigeration purposes.

5 Carbon dioxide snow is now generally produced from carbon dioxidewhich is maintained in liquid form by subjecting it to criticaltemperature and pressure of liquefaction, the liquid being transformedto'snow by releasing the pressure to permit sudden gasification of theliquid and expansion of the resultant gas. However, the known processesand apparatus for the production of carbon dioxide snow in this way havenot been entirely eflcient or satisfactory, and the commercialproduction and use of carbon dioxide snow for refrigeration purposes hastherefore Y been more or less limited.

As contrasted with the above, the present invention contemplates theburning of natural gas in the presenceY of air to support combustion,thereby producing flue gases which when dried, are primarily composed ofabout 10 percent carbon dioxide gas and about 90 per cent nitrogen.These dried flue gases are then compressed to a temperature of aboutminus 68 degreescenti grade, or slightly about the/solidication point ofthe carbon dioxide gas, whereupon-the compressed and cooled gases aredischarged into an expansion chamber. The sudden expansion of the gasesin the expansion chamber lowers their temperature below the solidicationpoint of the carbon dioxide gas, thus causing a substantial volume ofthe latter to solidify. The nitrogen, together with any unsolidiedcarbon dioxide, is allowed to discharge from the expansion chamber,thereby leaving the solidified carbon dioxide in said chamber, forrecovery as desired. By reason of my invention the carbon dioxide gasmay 40 be readily and cheaply producedun large quantities, and, due tothe separation of the carbon dioxide gas from the nitrogen by directsoldiiication and without any intermediate steps of liqueprovides forready and economical production of carbon dioxide snow in largequantities so as to permit its extensive use for refrigerationvpurposes. Moreover, the process may be readily carried out by the useof a simple and eflicient apparatus which is substantially continuous inoperation and capable of being Aproperly operated with a minimum amountof skill and attention.

The present invention further contemplates certainV novel featureswhereby the percentage of carbon dioxide gas transformed to snow may be'about 3,000 pounds per squar'inch and cooled to faction of the carbondioxide gas, the inventionl ing, two volumes of water vapor and onevolume nomically effected and readily fand properly controlled. A

The above and other features of my invention will be more evident fromthe following description inconrrection with the accompanying drawings,in which` f j Figure 1 is a diagrammatic elevation,-partly broken awayand in section, of an apparatus suit- 65 able for practice of myinvention. 1 Figure 2 is a diagrammatic plan view thereof. Figure 3 is avertical longitudinal sectional view of the expansion chamber; and

Figure 4 is a vertical transverse section on line l0 4 4 of Figure 3.

In burningl natural gas in the presence of air to support combustion,the uegases resulting can be said to be primarily composed of about 10per cent carbon dioxide (CO2) and 90 per cent 75 nitrogen (N2). The iluegases vary, of course, according to the exact nature of the natural gasvand the exact amounts of the hydrocarbon and inert gas constituentsthereof. However, for the purpose of this spcification, we may assumethat upon burning natural gas, the ue gases will be constituted of 10per cent carbon dioxide (CO2) and per cent nitrogen (N2) For a basis ofexplanation in arriving at the above mixture, it is pointed out thatnatural gas 85 is composed largely of the hydrocarbons of l methane(CI-I4) and ethane (02H8), the methane predominating in volume, althoughthe volumes of methane and ethane and the ratios of proportions of oneto the other will vary. 'I'he 90 phenominae of burning natural. gases isgoverned by their hydrocarbon content, but in any case the followingequations transpire:

From the first equation, it is apparent that to burn one volume ofmethane requires two volumes of oxygen, and as a result of the burnofcarbon dioxide are produced. As shown by the'second equation, three andone-half volumes of oxygen are required to burn one volume of ethaneyandas a result of the burning, three volumes of water vapor and two volumesof carbon dioxide are produced. f

By the use of air to support the combustion, oxygen is supplied andconsumed from it upon reaction with the hydrocarbon in the proportionshydrogen in a molecule of the hydrogen, form-A of one volume of oxygento each four atoms of ing water; and one volume of oxygen for each atomof carbon in the molecule of the hydrocarbon, forming carbonfdioxide.'I'he air or atmosphere is substantially 21 per cent oxygen and 79 percent nitrogen, by volume, although there are small amounts of helium,argon, etc., which, from a) practical standpoint, may be ignored.Therefore, upon consumption of the oxygen of a given Volume of airrequired to burn a given volume of natural gas, the oxygen is consumedin combination, forming water vapor and carbon dioxide, as shown above,and leaving the inert nitrogen in mixture with the water vapor andcarbon dioxide. Therefore, upon condensing and removing the water vapor,the flue gases will, upon what may be called an average, consist ofabout 10 per cent carbon dioxide and 90 per cent, nitrogen, by volume. y

As carbon dioxide and nitrogen do not follow the mixture rule, we eiecta separation of the carbon dioxide from the nitrogen by directsolidicationf or Without any intermediate steps of liquefaction of thecarbon dioxide. Y'

In the drawings, 5 indicates a furnace having a burner 6, supplied withnatural gas from a suitable source, and also with suicient air tofurnish the necessary oxygen to effect substantially complete combustionof the hydrocarbon constituents of the natural g'as. A pipe 7 isprovided to conduct the ilue gases from the furnace 5 through acondenser 8, in which the water vapor is con pressor 11, the flue gasesare placed under a pressure of about 3,000 pounds per square inch,further moisture being expressed from the gases therein and drawn off asnecessary.` The gases are then conducted by a pipe 12 to an absorbentdrier 13, where any remaining trace of moisture may be removed so thatit will not freeze in and clog the coils of the cooling apparatus 14 towhich the gases are conducted from the drier 13 by a pipe 15. The drier13 may consist of a cylinder packed with calcium chloride, or the like,through which the gases pass to have the moisture absorbed therefrom.

The 'cooling apparatus 14 is shown as including a double-walled casing17, having a packing 18 of heat-insulating material between the wallsthereof, and defining an enclosed cooling cham-` ber 19 in which isdisposed a cooling coil 20. The

Y pipe 15 has a valve 21 tocontrol the flow of flue y ber 19 and isprovided with a control valve 28.

A pressure gage 29 is connected with the pipe 27 at the inlet side ofvalve 28 by means of a branch pipe 30. Thermocouples or resistancethermometers 31 a-re arranged to indicate the temperatures whichmaintain at the point where the pipe 25 enters the .chamber 19, wherethe pipe 27 leads vthe pipe 44 enters chamber 39, where the pipe 45leads from chamber 39, and at the points where from chamber 19, and atthe points where the flue gases enter and pass from the coil 20.

The expansion or snow chamber 23 is provided along its upper surfacewith ltering facilities consisting in a filter cloth or screen 32attached 8U to a perforated metal sheet 33, enclosed within a trap cover34 which provides a space or gas trap 35. The exhaust pipe 25 connectswith the cover 26, providing a means of exhaust for gases from gas trap35.

Means is provided to super-cool the gases as they are discharged Iintoand expanded in the snow chamber 23, whereby the percentage of carbondioxide gas transformed' to snow will be greatly increased.Thissuper-cooling means per se forms no partof this, invention, butpreferably consists of a double-walled casing 37 having a packing 38 ofheat-insulating material between at its outlet end. Leading from thejacket 42 is 100 an outlet' pipe 44- which enters the bottom of chamber39. A further pipe 45 forms a top outlet for the chamber 39 and isprovided with a control valve 46. The pipe 45 leads to a compressor 47whose outlet line 48 connects with the 105 top of coil 40. A closedcircuit is thus provided through which is circulateda cooling agent,preferably ethylene gas. The ethylene g'as iscompressed toor below itscritical pressure, which, under ordinary circumstances, is about 787pounds per square inch. After compression, the ethylene gas enters thecoil 40 where it is-coold to 6r belowits critical temperature (about.minus 9.9 degrees centigrade), whereupon thegas liquees and dischargesthrough pipe 41 into the jacket 42. As the liquid ethylene dischargesinto the jacket 42, it expands and returns to gaseous form, therebyproducing a very low temperature in jacket 42 and super-cooling thegases discharged into the snow chamber 23. Thus evaporated back to a,gas, the ethylene gas passes through pipe 43 into and through thechamber 39 toA cool the ethylene gas in the coil 40, and then passesback to the compressor 47 through pipe 45. A coil 49, forming part of asmall cooling unit, may be provided to cool the ethylene gas afterleaving the compressor 47 and before entering the coil 40. When thissmall cooling unit is used, a considerably less pressure would have tobe exerted upon the ethylene gas for its transition to liquid than itscritical pressure. For instance, if the gas is cooled to minus 30degrees centigrade, a pressure of about 280 pounds per squareinch wouldsuflice. As this operates to cool the ethylene below its criticaltemperature, a given amount of power expended in compressing theethylene will produce a larger quantity of liquid ethylene to beevaporated in the jacket 42, thus increasing the efficiency of thesuper-cooler. i

Apressure' gage 50 is connected with the pipe 45 at the inlet side ofvalve 46 bymeans of a branch pipe 51. Thermo-couples or electricalthermometers 52 are arranged to indicate the temperatures which maintainat the point where tl liquid ethylene enters and passes from the coilA40. The valve 46 has the important function,

through its operation of providing a back-pressure in chamber 39 andjacket 42, to effectively 15o induce/heat transfer from the coil 40 tothe cold gas in chamber 39.

In operation, the volume of iiue gases entering the coil 20 under thepressure produced by previous compression to approximately 3,000 poundsper square inch, may be controlled, if desired, by f the valve 21.

The ue gases owing down through thecoil 20 pass therefrom into pipe 22,through which they are drawn oil from said coil. The gases are thenpermitted to discharge from pipe 22 through a small orifice into thesnow chamberv23, the discharge being controlled by means of the valve 24at the lower extremity of pipe 22.

In their downward passage through the coil 20,' the compressed gases areexceedingly cooled and, under the low temperature so obtained and thepressure exerted upon the gases, upon the discharge into the snowchamber, the carbon dioxide solidies to a substantial degree.

The cold nitrogen which remains uneffected other than being cooled to alow temperature, re-

turns upward under its own pressure from theA snow chamber 23 throughpipe 25, such return of the nitrogen being regulated as desired by valve26. The nitrogen then discharges from the pipe 25 into the chamber 19,passing upward therein gever the coil 20, and thereby cooling thedownwardly moving gass in said coil. The nitrogen then dischargesthrough pipe 27, being Aregulated in discharge by valve 28'. 'Ihe gauge29 constantly indicates the pressure in the chamber 19 and in the snowchamber 23. f

It is important that the valve 24 be at the lower extremity of pipe 22,for, by being so arranged, no solid carbon dioxide will resultin pipe221' Care should be exercised to maintain the temperature of the gasesin the coil 20 slightly above minus.'l0 C.'-which, under thecircumstances, is-

the approximatepoint oi'solidiflcation of carbon dioxide, as anysolidification occurring within the coilwould tend to clog it and impedeor hinder the movement of gases therein. Carbon dioxide separate from ahighly compressed mixture ofl nitrogen and carbon dioxide at temperaturewhere it would be expected to appear from the wellknown vapor pressureof liquid carbon dioxide and the rcalculated partial pressure of carbondioxide in a mixture. In experiments, no liquid carbon dioxide could beseen to separate from a highly compressed mixture of 90% nitrogen and10% carbon dioxide, but the latter first appeared as a solid.Apparently, the highly compressed nitrogen dissolved the liquid andsolid carbon dioxide to aconsiderable degree, the solution remaininggaseous.

The expansion of the gases passing from the lower extremity of pipe 22into snow chamber 23 through a snall orice; causes a rapid lexpansion ofthe volume upon its discharge into chamber 23, as well as evaporation ofa slight amount of liquid carbon dioxide, if any has resultedpreviously. This materially lowers the temperature of the alreadyv coldgases below the ,70 freezing'point of carbon dioxide within the snow beeilected in the snow chamber 23 and in chamber 19. An important functionof the back pressure in chamber 23 is to increase the eiiiciency of theprocess by making the partial pressure of the carbon dioxide, which isxed at any given temperature by the vapor pressure of the' .solid carbondioxide with which it is in equilibrium, a smaller fraction of the totalpressure and so decreasing the fraction of the carbon dioxide in theescaping gases. y

At each temperature up to the. melting` point of solid carbon dioxidethere is a certain pressure exerted by the vapor of carbon dioxide atwhich there is equilibrium between the solid and the vapor. Thisequilibrium pressure is called the vapor pressure of carbon dioxide. Atminus 78.3 C. it is one atmosphere. At minus 104 C. it is about onetenth of an` atmosphere. If the gaseous carbon dioxide, carbon dioxidevapor as we called it above. is mixed with some other gaseous substance,say nitrogen, each gas will exert its own individual or partial pressurenearly independent of the presence of the other and the total pressureof the mixture, that shown by a pressure gauge, will be equal to the sumof the partial pressures of the individual gases. These partialpressures are equal to the total pressure multiplied by the fraction ofthat gas by volume in the mixture. For example, if the total pressure ona mixture of 10% carbon dioxide and 90%- nitrogen is 150 lbs. thepartial pressure of the. carbon dioxide is one tenth of 150, or fteenlbs. and that of the nitrogen nine tenths of 150,or 135 lbs. Solidcarbon dioxide lwill be vin equilibrium with a mixture of gaseousnitrogen and gaseous carbon dioxide when the partial pressure ofthecarbon dioxide is equal to the vapor pressure of the solid carbondioxide. At minus 104 C; this is,` as stated above,- one tenth of anatmosphere and if the flue gases were allowed to expand to oneatmosphere while YVbeing cooled to minus 104 C., no solid carbon dioxidewould be deposited because the 10% of carbon dioxide would be justenough to give a partial pressure of one tenth of an atmosphere and'allof the carbon dioxide would escape from the snow chamber along with thenitrogen. Therefore the efficiency of the process under these conditionswould be zero. However, if at minus 104 C. a back pressure of 150 lbs.,or ten atmospheres, is maintained in the snow chamber the partialpressure of the carbon dioxide, if no condensation should take place,would be one tenth of ten or one atmosphere and, therefore,approximately nine tenths of. the carbon dioxide would separate as thesolid and only about one-tenth escape with the gaseous nitrogen; makingthe efiiciency of the process about 90%. Therefore, the effect of theback pressure is to increase the yield of solid carbon dioxide snow bydecreasing its partial pressure in the escaping gases. An-

eflectively utilize the cooling properties of the cold nitrogen toattain a lower temperature in the chamber 19 than would result i! thecold nitrogen was permitted to ilow under its own pressure therethrough.r

By means of thev gage 29, constantly indicating the back-pressure. theproper operation of the `valve 28 is made a matter of simplicity;

The resistance thermometers 3l, constantly indicating the temperature ofthe gas volume within the coil 20, and the temperature of the nitrogenentering chamber 19, respectively register the ascertainabletemperatures and their iiuctuations as occasioned by the operation of135" other functionof this 'back-pressure is to more valve 2s toincrease or diminisnthe back-presI Sure.

s'that should itbe found expedient to have a greater back-'pressure insnowchamber 23 than in chamber 19 under possible circumstances, the samecan be attained.

-If desired, the pipes and 27.and the pipes 43 and 45 may be associatedwith coils surrounding the coils 20 and 40, instead of providing theinsulated chambers 19 and 39. Other minor changes and substitution ofequivalents are contemplated within the spirit and scope of theinvention. as claimed.

What we claim as new is:

1. The method of making carbon dioxide snow from dried flue gasescontaining about 10% carbon dioxide and 90% nitrogen, which consists incompressing said dried ilue gases to about 3000 lbs. per square inch,cooling said dried andvcompressed flue gases to a temperature slightlyabove '-76" C., and discharging the compressed and -76 C.,dischargingthe compressed and cooled gases through a restricted outletinto a chamber to permit expansion and reduction in the pressure of thegases to about 150 lbs. per square inch' to further` cool said gases,and super-cooling said chamber as the gases are discharged and expandedtherein so as to further cool the gases to' and maintain them at about-104 C. after u, expansion of the same in said chamber to insuresolidication of about nine-tenths of the carbon dioxide.

JOSEPH S; BELT. l `HAMILTON P. CADY.

